Lithotripsy (stone-breaking) machines focus ultrasonic energy at stones (or other internal sites) for eroding the stone down to a size that can be passed by the patient. The ultrasonic energy is generated by piezoelectric crystal transducers mounted on a lens assembly. The transducers are internally stressed or "charged" by an intense electric field and discharged simultaneously to generate a collective pulse of ultrasonic energy.
Heretofore, the crystal transducers were mounted on the back of a flat plate lens assembly which provided a flat mounting face for the transducers. The front of the lens assembly was a single large concave surface designed to focus the ultrasonic energy from all of the transducers at an invivo point within the patient. The simultaneous firing of the transducers promoted an "in phase" relationship between the individual pulse from each transducer, thereby increasing the pulse intensity of the collective pulse.
However, transducers near the edge of the flat plate lens assembly had a longer ultrasound path length to the target region than transducers near the center. As a result of this edge delay, the energy pulses from the peripheral transducers arrived at the target region later. The resulting phase loss reduced the intensity of the collective pulse. A corrective curve for the lens assembly having a generally elliptical or oblate shape was required to correct this peripheral astigmatism. The edge delay could not be corrected by using a simple concave curve with a true spherical shape.